TeamWin "No OS Installed" Android Nexus 5

[AC Question]

I lost my phone in Peru and had to downgrade to my old Nexus 5. It was rooted so I couldn't update the OS. In trying to unroot it, I [somehow] deleted the OS, and now the phone won't boot. I get to the TeamWin recovery options and it will say "No OS Installed, are you sure you want to..." How can I fix this?
Note: nexus 5 does not allow for external SDs to transfer files, etc.

 

[B. Diddy]

Moved to the Nexus 5 Rooting forum.

 

[Golfdriver97]

Welcome to the forums. You would have to reflash the stock ROM through fastboot and your PC.